The present invention relates to a wafer probe for high frequency testing of integrated circuits and other electronic devices.
Integrated circuits (ICs) are economically attractive because large numbers of often complex circuits, for example microprocessors, can be inexpensively fabricated on the surface of a wafer or substrate. Following fabrication, individual dies, including one or more circuits, are separated or singulated and encased in a package that provides for electrical connections between the exterior of the package and the circuit on the enclosed die. The separation and packaging of a die comprises a significant portion of the cost of manufacturing a microelectronic device. To monitor and control the IC fabrication process and to avoid the cost of packaging defective dies, manufacturers commonly add electrical circuits or test structures to the wafer to enable on-wafer testing or “probing” to verify characteristics of the integrated circuits before the dies are singulated.
A test structure or device-under-test (DUT) typically comprises a simple circuit that includes a copy of one or more of the basic circuit elements of the integrated circuit, such as a single line of conducting material, a chain of vias or a single transistor. The circuit elements of the DUT are typically produced with the same processes and in the same layers of the fabrication as the corresponding elements of the marketable integrated circuits. Since the circuit elements of the DUT are fabricated with the same processes as the corresponding elements of the integrated circuits, the electrical properties of the DUT are expected to be representative of the electrical properties of the corresponding components of the integrated circuits. In addition to the DUT, test structures typically include a plurality of metallic contact or probe pads that are deposited at the wafer's surface and a plurality of conductive vias that interconnect the probe pads and the subsurface DUT. The performance of the test structure is typically tested by applying a test instrument generated signal to the probe pads and measuring the response of the test structure to the signal.
At higher frequencies, on-wafer characterization is commonly performed with a network analyzer. The network analyzer comprises a source of an AC signal, commonly a radio frequency (RF) or microwave frequency signal, that is transmitted to the DUT to produce a response. A forward-reverse switch directs the stimulating signals toward one or more of the probe pads of the DUT where a portion of the signal is transmitted to the DUT and another portion is reflected. Directional couplers or bridges pick off the forward or reverse waves traveling to and from the DUT. The waves are down-converted by intermediate frequency (IF) sections of the network analyzer where the signals are filtered, amplified and digitized for further processing and display. The result is a plurality of s-parameters (scattering parameters), the ratio of a normalized power wave comprising the response of the DUT to the normalized power wave comprising the stimulus supplied by the signal source.
The preferred interconnection between a network analyzer or other test instrument and a DUT is a wafer probe typically comprising a movable body that supports one or more contacts or probe tips that are arranged to engage a test structure's probe pad(s) on the surface of a wafer. Burr et al., U.S. Pat. No. 5,565,788, disclose a microwave probe comprising a support block which is attachable to a movable probe supporting member of a probe station. The support block supports a first end portion of a section of coaxial cable. The second end of the coaxial cable is freely suspended and, in turn, supports a probe tip section. Integrated circuits commonly utilize single ended or ground referenced signaling with a ground plane at the lower surface of the substrate on which the active and passive devices of the circuit are fabricated. Although there are a number of potential arrangements for the probe pads of a test structure that utilizes single-ended signaling, a common arrangement places a signal probe pad between a pair of spaced apart, grounded probe pads, a so-called ground-signal-ground (GSG) arrangement. The tip section of the microwave probe disclosed by Burr et al. includes a central signal conductor and one or more ground conductors generally arranged parallel to each other in a common plane to form a controlled impedance structure. The signal conductor is electrically connected to the inner conductor of a coaxial cable and the ground conductors are electrically connected to the outer conductor of the cable at the freely suspended end of the cable. A shield member is interconnected to the ground conductors and covers at least a portion of the signal conductor on the bottom side of the tip section. The shield member is tapered toward the tips with an opening for the tips of the conductive fingers. The signal conductor and the ground conductors each have an end portion extending beyond the shield member and, despite the presence of the shielding member, the end portions are able to resiliently flex relative to each other and away from their common plane so as to permit probing of devices having non-planar probe pad surfaces.
In another embodiment, Burr et al. disclose a microwave probe that includes a supporting section of coaxial cable including an inner conductor coaxially surrounded by an outer conductor. A tip section of the microwave probe includes a signal line extending along the top side of a dielectric substrate connecting a probe finger with the inner conductor of the coaxial cable. A metallic shield may be affixed to the underside of the dielectric substrate and is electrically coupled to the outer metallic conductor. Ground-connected fingers are placed adjacent the signal line conductors and are connected to the metallic shield by way of vias through the dielectric substrate. The signal conductor is electrically connected to the inner conductor and the ground plane is electrically connected to the outer conductor of the coaxial cable. The signal conductor and the ground conductor fingers (connected to the shield by vias) each have an end portion extending beyond the shield member and, despite the presence of the shielding member, the end portions are able to resiliently flex relative to each other and away from their common plane so as to permit devices having non-planar contact surfaces to be probed. While the structures disclosed by Burr et al. are intended to provide uniform results over a wide frequency range, they unfortunately tend to have non-uniform response characteristics at higher microwave frequencies.
Gleason et al., U.S. Pat. No. 6,815,963 B2, disclose a probe comprising a dielectric substrate that is attached to a shelf cut in the underside of the probe tip supporting portion of coaxial cable. The substrate projects beyond the end of the cable in the direction of the longitudinal axis of the cable. A signal trace is formed on the upper side of the substrate and conductively connects the center conductor of the coaxial cable with a via at the distal end of the signal trace, near the distal edge of the substrate. The via, passes through the substrate and conductively connects the signal trace to a contact bump or tip that will be brought into contact with a probe pad of the test structure to enable communication of a single-ended signal from the center conductor of the coaxial cable to the DUT. A conductive shield which is preferably planar in nature is affixed to the bottom surface of the substrate and electrically connected to the outer conductor of the coaxial cable. The conductive shield is typically coextensive with the lower surface of the substrate with the exception of an aperture encircling the contact tip for the signal trace. Contacts for contacting probe pads connected to the ground plane and spaced to either side of the signal probe pad of the test structure may also be provided. The conductive traces comprise a coplanar waveguide when the probe is operated at microwave frequencies.
At frequencies between DC and approximately 60 gigahertz (GHz), a coaxial cable is frequently preferred for communicating signals between the test instrumentation and the DUT. However, the central signal conductor of a coaxial cable is relatively small and, at microwave frequencies, skin effect restricts the current carrying area of the conductor to a thin layer at the conductor's surface. Heating of the dielectric separating the signal conductor and the coaxial ground conductor, may further impede the transmission of the signal. At frequencies greater than 40 GHz the probe and the test instrument are commonly interconnected with a waveguide. The waveguide comprises a hollow tube of conductive material, often rectangular in shape. Electromagnetic waves propagate in the waveguide channel by successive reflections from the inside surface(s) of the wall defining the waveguide channel. A waveguide is characterized by excellent isolation between signals and very low loss.
While a waveguide provides a low loss path for communicating signals between a probe and the related test instrumentation, the probes of Burr et al. and Gleason et al. rely on coaxial cable for connecting the waveguide and the probe tip. In addition to the reduced transmission efficiency of the coaxial cable resulting from skin effect and dielectric heating, the transition from coaxial cable to waveguide can be difficult and can introduce a substantial loss of signal. The transition from the waveguide to the coaxial cable is commonly accomplished by inserting the tip of the coaxial cable's center conductor into the interior of the waveguide and connecting the outer conductor to the wall of the waveguide. The projecting conductor acts as an antenna for the signals propagating in the waveguide. In a typical implementation a backshort, usually made of brass or some other reflective material and having a reflective face, is also included in the waveguide channel. The backshort is preferably located close to the projecting center conductor and typically oriented perpendicular to the waveguide channel so as to reflect any alternating signal present within the waveguide channel towards the projecting conductor. If properly positioned, the backshort will reflect the alternating signal within the waveguide into a standing wave pattern and signal degradation will be minimized in the transition from the coaxial cable to the waveguide. However, adjusting the position of the backshort relative to the center conductor of the coaxial cable to optimize performance in the primary band of the alternating signals, is often difficult and at high frequencies, very small deviations from the optimal position of the backshort may lead to significant signal degradation.
Katoh, U.S. Pat. No. 5,408,188, describes a wafer probe for high frequency single-ended signals in which a waveguide transitions directly to a coplanar line at the probe tip. The probe tip comprises a dielectric blade having a centrally located (laterally) signal conductor affixed to the lower surface of the blade. Ground conductors, spaced apart on either side of the signal conductor, are also affixed to the lower surface of the blade. The blade is clamped between separable upper and lower portions of the waveguide with the ground conductors in contact with the lower interior surface of the waveguide's wall. A stepped ridge affixed to the upper interior surface of the wall extends downward in the waveguide to approximately the level of the lower surface of the dielectric blade and the signal conductor which is affixed to the lower surface of the dielectric blade. The signal conductor is conductively interconnected with the downward projecting ridge. The high frequency waveguide probe enables probing with high frequency, single ended signals through the commonly utilized ground-signal-ground probe pad arrangement. However, the probe requires a special, split, waveguide section that includes a downwardly projecting ridge in the interior of the waveguide and, according to Katoh, the transition from the coplanar line of the probe tip to the waveguide can result in significant transmission losses.
While single-ended or ground referenced signaling predominates at lower frequencies, the integrity of single-ended signals is jeopardized at higher frequencies. Integrated circuits are fabricated by successive deposition of conductive, semi-conductive and insulating materials on a semi-conducting wafer and, as a result, electrical interconnections commonly exist between parts of the circuit's devices and between the devices and the substrate on which they are fabricated. These interconnections are commonly capacitive or inductive in nature resulting in frequency dependent parasitic impedances that make the true nature of ground referenced signals uncertain as the operating frequency of the circuit increases.
Referring to FIG. 9, a balanced device 100 comprises two nominally identical circuit halves 100A, 100B that are “virtually” grounded 104 at the symmetrical axis 102 of the two circuit halves. A balanced device outputs a differential mode signal comprising even and odd mode components of equal amplitude and opposite phase (So+1 and So−1) 106A, 106B when stimulated with a differential mode signal comprising even and odd mode components of equal amplitude and opposite phase (Si+1 and Si−1) 108A, 108B. The virtual ground is independent of the physical ground path of the IC and, at the operating frequency, the potential at the virtual ground does not change with time regardless of the amplitude of the stimulating signal. Balanced devices have become increasingly attractive as the operating frequencies of ICs have increased because the virtual ground enables balanced or differential circuits to tolerate poor RF grounding better than circuits operated with single-ended signals.
What is desired, therefore, is a low cost wafer probe enabling efficient communication of high frequency, differential signals between a DUT and a test instrument.